1. Field of the Invention
The present invention relates to an image forming apparatus having a colorimetric function.
2. Description of the Related Art
In an image forming apparatus, the quality of an image is determined based on graininess, in-plane uniformity, character quality, and color reproducibility (including color stability). In the recent spreading of the multi-color image forming apparatus, the color reproducibility is sometimes referred to as the most material factor for determining the quality of an image.
Each person has a memory of colors (e.g., specifically, colors of human skin, blue sky, and metal) he expects based on his experience. The person will have an uncomfortable feeling when seeing a color beyond a permissible range of the color he expects. Such colors are called “memory colors”. The reproducibility of the memory colors are often expected when photographs are output.
A demand for a good color reproducibility (including color stability) is increasing with respect to the image forming apparatus. For example, in addition to the photo-images, there are office users who have uncomfortable feeling of difference in colors between a document image on a monitor and an actual document, and graphic art users who pursuit the color reproducibility on a computer-generated (CG) image.
To satisfy the good color reproducibility demanded by the users, for example, Japanese Patent Application Laid-Open No. 2004-086013 discusses an image forming apparatus for scanning a measurement image (i.e., a patch image) formed on a recording paper by using a color sensor provided in a conveyance path for conveying the recording paper.
In the image forming apparatus, the measurement image is formed on the recording paper by using toners and a scanning result of the measurement image measured by the color sensor is fed back to processing conditions such as an amount of exposure and developing bias, thereby enabling reproduction of a density, gradation, and a tint to some extent.
However, in the image forming apparatus discussed in Japanese Patent Application Laid-Open No. 2004-086013, since the color sensor is disposed in the conveyance path in the vicinity of a fixing device, and chromaticity of the measurement image as a measuring object varies according to a temperature. This phenomenon is called “thermochromism”. The “thermochromism” is induced such that a molecular structure forming a color material such as toner and ink is changed according to “temperature”. Note, in the description below, the word “chromaticity” is just being used to express color. A particular chromaticity value may be expressed in the L*a*b color space. Other color spaces may be used without going beyond the scope and spirit of the invention as recited in the claims. Also, the “chromaticity value” is equivalent to the “color value”.
In order to measure a color of the measurement image within the image forming apparatus, the color measurement is to be performed after the color material is placed on the recording paper and in a state where the color materials are mixed on the recording paper. In the image forming apparatus using inks as color materials, the color is required to be measured after the color materials are dried by heat by using a drying device. In the image forming apparatus using toners as the color materials, the color is required to be measured after the toners are heated and fused to be mixed by a fixing device. Therefore, the color sensor needs to be placed downstream of the drying device and the fixing device in a conveyance direction for conveying a recording paper.
On the other hand, in order to form the image forming apparatus in a compact size, a length of the conveyance path from the drying device and the fixing device to the color sensor needs to be as short as possible. Therefore, the recording paper and the color materials heated by the drying device and the fixing device are conveyed to the color sensor without being cooled to a room temperature. A temperature of the recording paper becomes higher than the room temperature due to a temperature rise in members within the image forming apparatus such as a conveyance guide of the recording paper or an atmospheric temperature rise within the image forming apparatus.
As described above, in the image forming apparatus equipped with the color sensor therein, a colorimetric measurement result which is different from the chromaticity under normal environment (i.e., under room-temperature environment) may be obtained due to an adverse effect of the thermochromism.